1. Technical Field of the Invention
The present invention relates in general to the mobile telephony field and, in particular, to a method for avoiding unnecessary signalling in a cellular communications system.
2. Description of Related Art
In certain cellular communications systems, such as for example, the Personal Digital Cellular (PDC) System in Japan, and the Pan-European Global System for Mobile Communications (GSM), selected information associated with mobile subscribers is stored at a switching node, such as for example, a PDC System's Visited Mobile Services Switching Center (VMSC), or a GSM's visited MSC/Visitor Location Register (MSC/VLR). In the GSM, the MSC and VLR are physically integrated but their functions are logically separate. A VMSC (or MSC/VLR) is typically responsible for setting up and terminating calls between system users and/or subscribers, and providing switching and location management functions for the subscribers' MSs located within the VMSC's service area.
For location management purposes, a VMSC maintains information in a database (typically in an associated Global Location Register or GLR) about all of the mobile subscribers currently located within that VMSC's service area. Notably, the same function is performed by a VLR in the GSM. Essentially, the VMSC maintains a copy of a portion of the mobile subscriber data stored in the Home Location Register (HLR) for all of the subscribers located within that VMSC's service area. As such, both the HLR and VMSC store location information for the subscribers located in that VMSC's service area.
In such cellular systems as the PDC System and GSM, a Public Land Mobile Network (PLMN) network portion typically includes several VMSCs (or MSC/VLRs). For example, FIG. 1 is a simplified block diagram of an exemplary cellular system 100. Notably, FIG. 1 can represent a PDC System, but it can also represent a GSM, Digital-Advanced Mobile Phone System (D-AMPS), or any other similar type of cellular system including an HLR or similarly functioning node. Although the cellular system 100 illustrated by FIG. 1 shows only two VMSCs (106, 108), it may be assumed that the system (100) can also include up to several other similarly-arranged VMSCs. System 100 also includes a Gateway MSC (GMSC) 102, which functions as a link between a PDC PLMN network and another PLMN, Public Switched Telephone Network (PSTN), and/or Integrated Services Digital Network (ISDN) network.
Notably, the radio network coverage areas of the VMSCs (e.g., 106, 108) overlap to a certain extent at their mutual border (e.g., near the border between Location Areas 1 and 2). Such an overlap of radio coverage is normal and in accordance with existing cellular system standards. However, a significant problem arises due to such radio network coverage overlap between neighboring VMSCs.
For example, a VMSC (e.g., 106) can experience a major hardware and/or software failure, and its radio network (e.g., control channel) may be temporarily turned off as a result. Additionally, certain failures can affect the VMSC's database so that some or all of the stored mobile subscriber data is lost. At the onset of a VMSC failure, all of the MSs powered on in that VMSC's radio network border area (e.g., near the border in Location Area 1) that can "hear" a neighboring VMSC's (e.g., 108) base station transmissions, will attempt to re-register and update their locations with that "live" VMSC (108). In a PDC or GSM network, the MSs typically conduct a Location Registration (location updating procedure) with the new VMSC (108). Usually, these MS Location Registrations with a new VMSC all occur within a relatively short period of time (e.g., only a few seconds).
Nevertheless, there can be tens of thousands of MSs powered on in congested urban areas near the border of the radio network coverage area for the failed VMSC (e.g., thousands of hand-held phones awaiting a page or engaged in a call in the Tokyo metropolitan area). Consequently, at the onset of the VMSC's failure (e.g., loss of the control channel), these thousands of MSs can send a location updating request (Location Registration) to a "live" neighboring VMSC (108, etc.) As such, each MS establishes a radio connection with, and then transmits a Location Updating Request message to, the "live" neighboring VMSC. Primarily, this message contains information needed to identify the MS's subscriber.
In a PDC system, when the "live" VMSC receives the Location Updating Request message, it checks the Mobile Station Identity (MSI) field in the message to determine whether that MS is registered with that VMSC (i.e., the MS has a record with subscriber data maintained at that VMSC) If the MS is "new" and not registered with that VMSC, then the VMSC initiates a location updating procedure with the HLR (104).
The VMSC (108) invokes the location updating procedure with the HLR (104) for each "new" MS, and sends the MSI and Pursuit Routing Number (PRN) associated with each such MS to the HLR. This information is conveyed from the VMSC to the HLR via the CCITT Common Channel Signalling System No. 7 (CC7) network signalling links in the PDC, or from the MSC/VLR to the HLR via the CCITT Signalling System No. 7 (SS7) network signalling links in the GSM.
A significant problem related to this location management approach is that since the Location Registration procedure can be invoked between the "live" VMSCs and the HLR for thousands of MSs, the HLR will receive a mass of location registration update messages over the CC7 (SS7) signalling links. Under normal conditions, if the original VMSC (106) had been operational, then the HLR would have sent a "Location Erasure" message to that VMSC to erase the old location information stored for the MSs concerned. This message is called a "Location Cancellation Message" in the GSM. However, since the original VMSC (106) has failed, the signalling connection between it and the HLR is inoperative. Consequently, the HLR has to store the Location Erasure information for all of the MSs involved in a queue, and as soon as the operation of the original VMSC (106) and signalling links is restored, send the pending mass of Location Erasure messages over the signalling links to that VMSC. Note that a signalling link failure can also occur for a number of reasons other than a VMSC's failure (e.g., a temporary failure of the signalling link itself). Consequently, it is necessary to be capable of buffering the Location Erasure messages at least temporarily in the HLR.
An existing approach used is to queue the pending Location Erasure messages in a buffer location in the HLR. Then, as the "failed" VMSC and signalling links are returned to service (e.g, operation of the original control channel is restored), the HLR transmits the pending Location Erasure messages to that VMSC. In response, the VMSC transmits a Location Erasure Acknowledgment message back to the HLR. Albeit, this solution is deemed practical and relatively easy to implement for relatively small network applications. However, for much larger applications (e.g., a cellular network in a congested metropolitan area such as Tokyo), the existing approach results in a mass of Location Erasure messages and Location Erasure Acknowledgment messages being sent via the (CC7/SS7) signalling network between the HLR and VMSC during a relatively short period of time (if the failed VMSC has lost its mobile subscriber data). More importantly, this mass message signalling ties up the VMSC's and signalling network's valuable processing resources, especially when the VMSC needs them the most (i.e., while the VMSC is attempting to recover from its serious failure). Nevertheless, as described in detail below, the present invention successfully resolves these and other related problems.